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Earth and Space 6-8 / Week 05 / Wednesday
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Week 05 · Gravity Holds the System

Wednesday

Sky Deck Lab: Orbit in a bowl
// One pull keeps the whole solar system together
⏱ about 20 min

Wednesday: Sky Deck Lab: Orbit in a Bowl

Rocket sets the biggest mixing bowl on the Sky Deck table. He drops a marble in and gives it a push along the side.

The marble races around the bowl in circles, slowly sinking lower and lower until it stops at the bottom.

"Orbit!" Rocket says. "Then it fell in. Real planets do not fall into the Sun."

Raven leans over. "What do you notice? The marble slowed down every lap. What slowed it?"

"The bowl rubs against it," Rocket says. "Friction. Space has almost nothing to rub against."

Nova hovers over the rim. "A model shows some things well and some things wrong. Your job is to tell which is which."

Your mission

Today you build an orbit model with a bowl and a marble. The bowl stands for the Sun's pull. The marble stands for a planet.

A faster push sends the marble higher up the side, in a wider circle. A slower push keeps it low and close.

Then you test the model against the real thing and record what it gets right and what it gets wrong.

  • One large smooth bowl, the bigger and rounder the better
  • One marble or a small smooth ball
  • A towel to set the bowl on so it does not slide
  • A timer or a clock with seconds
  • Your Sky Log and a pencil
Safety first
Marbles are small. Keep them away from younger children and pets, and never put one in your mouth.
Work at a table over a towel. Pick up any marble that escapes right away so no one slips.
Ask a grown-up before borrowing a bowl, and carry it with both hands.
  1. Set the bowl on the towel. Hold the marble against the inside wall, about halfway up.
  2. Give it a gentle push along the wall. Count how many laps it makes before it stops. Record the number.
  3. Repeat with a slightly harder push. Record the laps again and notice how high on the wall it rides.
  4. Repeat with a soft push. Does the marble stay low or ride high? Record it.
  5. Watch one run closely. Does the circle get bigger or smaller as the marble slows? Write it down.
PushLaps before it stoppedHigh or low on the wallCircle got bigger or smaller
Gentle
Harder
Soft
PREDICT, THEN CHECK
  • Read the question.
  • Tap your answer.
Before you push: which marble will ride higher on the bowl wall?
Why does the real Moon not slow down and fall in, like the marble?

What the model gets right and wrong

Right: the marble curves because something pulls it toward the middle. For a planet, that pull is the Sun's gravity.

Right: a faster planet can ride in a wider orbit, just as a faster marble rides higher on the wall.

Wrong: the marble slows because it rubs against the bowl. Planets move through almost empty space and keep going.

Wrong: the marble finally falls to the bottom. Earth has been falling around the Sun for about 4.5 billion years without falling in.

Model claimTrue or false?
The bowl stands for the Sun's pull toward the center.?
A harder push made the marble ride higher on the wall.?
Real planets slow down and fall into the Sun after a few laps.?
A model can be useful even when parts of it are wrong.?
WHY THIS EXERCISEScientists test every model against the real system. Knowing its limits is part of using it well.
ONE RUN OF THE MARBLE
  • Tap a card.
  • Then tap its spot.
1First
2Second
3Third
4Fourth
Try this
Try the model with a smaller ball or a bead. Does it make more laps or fewer? Record what changes.
Tilt the bowl very slightly on a folded towel. What happens to the circle? Set it level again when done.
Draw your bowl from the side. Mark where the marble rode after a hard push and after a soft push. Label the Sun at the bottom.

Careful work. Tomorrow you read a NASA table and find out why you would weigh less on Mars.

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